1. Technical Field of the Invention
The present invention relates generally to nondestructive evaluation of imperfections in ceramics and more particularly to an acoustic waveguide technique for detecting delaminations, voids and microcracks.
2. Discussion of the Related Art
At present, there is a need to develop improved technology for mounting very high speed electronic components in order to design circuits more precisely. That need has been partially fulfilled by the advent of surface mount chip capacitors. The presence of critical defects can cause premature failure. These defects can develop in the individual engineering components under normal operating conditions. Numerous inspection techniques have been developed for the detection of small and embedded defects that provide information about the integrity of the electronic materials. Nondestructive techniques are used to inspect the quality of production at the early stage of manufacture. The first stage of inspection is visual, and is suited only for relatively large external defects. This visual inspection can be enhanced by using microscopes to detect small and embedded defects.
A number of nondestructive inspection systems are being considered for inspecting multilayer ceramic capacitors (MLCC). The various testing methods used to investigate the MLCC are acoustic emission, X-ray radiography, neutron radiography, and acoustic microscopy. These techniques each possess disadvantages. Acoustic emission requires a large load to obtain good sensitivity which might cause flaws in the sample. X-ray or neutron radiography is unable to detect the small cracks in the sample. Acoustic microscopy is very expensive and difficult to use outside the laboratory.
U.S. Pat. No. 4,344,326 to Kahn describes a technique for testing of laminated capacitors using acoustic emission. The use of acoustic emission is based upon the premise that defects such as cracks and delaminations will propagate with the release of acoustic energy when an external stress is applied to a defective capacitor. In this technique, a capacitor is placed on a planar base and a ram applies a force in a direction perpendicular to the laminations of the capacitor. Any acoustic energy signals emanating from the capacitor under test will be transmitted through the ram, detected by an attached transducer and forwarded to an acoustic emission processor. The acoustic energy emitted by the application of the load can be detected, quantified and used as an indication of the presence and the severity of the physical defects.
An acoustic waveguide technique has been employed to observe the sensitivity of the acoustic amplitude of a propagated wave to the epoxy state and temperature of an epoxy. The wave is propagated through a copper wire embedded in the epoxy. The use of an acoustic waveguide is promising because it does not need any electrical contacts. This technique is described in "Propagation of Acoustic Waves in a Copper Wire Embedded in a Curing Epoxy," K. J. Sun and W. P. Winfree, Proceedings of IEEE Ultrasonics Symposium, pp. 439-442, 1987.
It is accordingly an object of the present invention to detect imperfections such as delaminations, voids and cracks in ceramics structure including multilayer ceramic chip capacitors.
It is another object of the present invention to accomplish the foregoing object nondestructively and noninvasively.
It is a further object of the present invention to achieve the foregoing objects using an acoustic waveguide technique.
Additional objects and advantages of the present invention are apparent from the drawings and specification which follow.